Can't change positioning of item after keyframe
The keyframe fixes the property (in this case the position) not only for a certain frame but also influences the future ans past values of that property. By adding only one keyframe that value is fixed for all time on that property. (think of this as a piecewise defined function). 

Your solution should be to add a new keyframe i on the frame you want to change the position. This should turn the property field yellow and you can set a new value for that frame. In the sidebar you can change the transition from one value to another over time.

No keyframe: the values are grey


keyframe on current frame (frame 0), values are yellow

frame 12 without keyframe, but a keyframe at frame 0 value is green (and cannot be permanently changed)

changed value and added new keyframe at frame 12. Values are yellow

frame 6 without keyframe, between frame 0 and 12, value is green and interpolated between these frames


Tldr: to have multiple values over time for one property you need multiple keyframes.